Planar transformer

ABSTRACT

An integrated transformer structure is disclosed. In one embodiment, a circuit is disclosed for a transformer circuit including two metallization layers and other layers, a conducting spiral, a gapped spiral, two radial conductors and various short bridging conductors and vias. Superior transformer price/performance is obtained without the use of more than two metallization layers. The invention may be embodied in GaAs and/or other technologies.

RELATED APPLICATIONS

None.

FIELD OF THE INVENTION

The invention generally relates to electronic circuits. The invention more particularly relates to inductors, especially transformers and autotransformers embodied in semiconductor based integrated circuits.

BACKGROUND OF THE INVENTION

Modern designs for analog electronic circuits are embodied as ICs (integrated circuits) and face considerable design challenges. As compared with other electrical elements (diodes, transistors, resistors, capacitors, etc.), inductors are more difficult to form in integrated circuits with good cost/performance. Inductors present particular difficulties because they may require relatively large areas and/or numbers of layers to achieve desired values of inductance. The traditional (discrete component) three-dimensional helical inductor shape is more or less impossible to realize within integrated circuits that are essentially two-dimensional in character.

It is desirable to keep the number of metallization layers to a minimum, especially in GaAs processes where often only two metallization layers are economically available.

The disclosed improved circuit designs for superior inductive circuit elements are capable superior tradeoffs between circuit performance and cost.

SUMMARY

Accordingly, the invention provides transformer and other circuits with superior performance and which may be implemented as an IC (integrated circuit) with semiconductor technologies such as GaAs (Gallium Arsenide) or InP (Indium Phosphate). Other semiconductor devices or totally different technologies may also be used.

A new topology/layout for designing spiral transformers (or mutually coupled spiral inductors) in integrated circuits (IC) processes which require as few as two metal/interconnect layers is disclosed.

According to an aspect of the invention, a circuit is disclosed for a transformer circuit including two metallization layers and other layers, a conducting spiral, a gapped spiral, two radial conductors and various short bridging conductors and vias.

Several variants of these aspects are also discussed together with alternative exemplary embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention:

FIGS. 1A, 1B and 1C show views of a prior art planar transformer.

FIG. 2 shows a plan view of a one layer of metallization of a transformer according to an embodiment of the invention.

FIG. 3 shows a plan view of a second layer of metallization of a transformer according to an embodiment of the invention.

FIG. 4 shows a perspective view, from an upper viewpoint of an exemplary transformer according to an embodiment of the invention.

FIG. 5 shows a further perspective view, from lower viewpoint of the exemplary transformer of FIG. 4.

For convenience in description, identical components have been given the same reference numbers in the various drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, for purposes of clarity and conciseness of the description, not all of the numerous components shown in the schematics and/or drawings are described. The numerous components are shown in the drawings to provide a person of ordinary skill in the art a thorough, enabling disclosure of the present invention. The operation of many of the components would be understood and apparent to one skilled in the art.

FIGS. 1A, 1B and 1C show a prior art planar transformer. The planar transformer of FIGS. 1A, 1B and 1C uses one metal layer (such as a top metal layer) for routing of both primary and secondary windings of the transformer. The primary and secondary windings are formed as two spiral inductors, more particularly as two intertwined spiral inductors. An exemplary top metallization layer is shown as FIG. 1A.

In the transformer of FIG. 1, each turn of the secondary winding spiral inductor is placed in between two adjacent turns of the primary winding spiral inductor and visa versa except for the first outermost turn of the primary winding and the last innermost turn of the secondary winding. This typically makes it necessary to use a conductor in a third dimension, such as using a second metallization layer to make electrical contact to the innermost ends of the respective spiral inductors. FIG. 1B shows the use of a lower metallization layer to access the inner port contact. FIG. 1C shows a cross section view of the same exemplary transformer.

A potential shortcoming of circuits having a topology similar to that of FIGS. 1A, 1B, 1C is that the self inductance of each inductor is relatively smaller due to the increased separation between two adjacent turns of each inductor. The closer the two adjacent turns of an inductor are the higher the self-inductance will be proportionately. Another potential disadvantage of circuits having a topology similar to that of FIGS. 1A, 1B, 1C is the inherent asymmetry of the design, which results in very unequal self-inductances of the primary winding versus the secondary.

Each inductor in the example shown in FIGS. 1A, 1B, 1C may, for example, have four full turns and an inner port contact. Referring to FIGS. 1A, 1B, 1C, one spiral conductor 1 makes up most of the primary winding and another spiral conductor 3 makes up most of the secondary winding. Radial conductors 21, 23 form part of the primary and secondary windings respectively. Via 11 electrically connects primary spiral 1 to radial conductor 21 to form the primary circuit. Via 13 electrically connects secondary spiral 3 to radial conductor 23 to form the secondary circuit.

An improved transformer conforming to the invention may be embodied without exceeding the desirable limit of two metallization layers. FIG. 2 shows a plan view of a one layer of metallization of a transformer according to an embodiment of the invention. FIG. 3 shows a plan view of a second layer of metallization of a transformer according to an embodiment of the invention. The transformer of FIGS. 2 and 3 may be fabricated using as few as two metallization layers, together with the a number of interconnects which may typically be fabricated as metallic vias or other means well known in the art.

Referring to FIG. 2, a spiral conductor 201 having an endpoint 201E in the upper the metallization layer 200 is shown and forms most of the primary winding of the transformer. Primary via 251 connects the spiral conductor 201 to the lower metallization layer which is shown in FIG. 3.

Still referring to FIG. 2, also formed in the upper metallization layer 200 are a number of short conductors that form bridges 202, each having, typically, two secondary vias 252. The short conductors 202 and secondary vias 252 form part of the secondary winding.

Referring to FIG. 3, a radial conductor 351 is formed in the lower metallization layer 300 and forms part of the primary winding of the transformer. The radial conductor 351 is connected to the primary via 251.

Several other structures in the lower metallization layer 300 are collectively arranged as a gapped spiral 363. Another radial conductor 362 connects to the gapped spiral 363, thus forming most of the secondary winding of the transformer. Portions of the gapped spiral 363 connect to the secondary vias 252 and hence to the upper metallization layer.

Thus, referring both to FIGS. 2 and 3, the primary winding of the transformer may be composed of spiral conductor 201, the primary via 251, and radial conductor 351. Similarly, the secondary winding of the transformer may be composed of gapped spiral 363, radial conductor 362, bridges (short conductors) 202, and second vias 252. The bridges serve to electrically interconnect the parts of the gapped spiral by bridging across the radial conductors.

FIG. 4 shows a perspective view, from an upper viewpoint of an exemplary transformer according to an embodiment of the invention.

FIG. 5 shows a further perspective view, from lower viewpoint of the exemplary transformer of FIG. 4.

Transformers created as embodiments of the invention such as those disclosed above have a number of advantages, over previously developed solutions. For example, the separation between adjacent turns of the inductors is less than with transformers embodied using previously developed solutions. This allows a higher self inductance for a given area or, alternatively, a small real-estate and hence a lower total cost to fabricate a given self inductance. Primary and secondary windings may be interchanged within the general scope of the invention as with most transformer designs.

Also, primary and secondary windings may have the same area and may be placed physically aligned when one on top of the other. This allows a higher degree of coupling between the windings than may be found in transformers according to previously developed solutions. Indeed the use of a thin substrate, such as a GaAs (Gallium Arsenide) substrate, may allow a very high degree of coupling notwithstanding these substantial capacitance so formed. Using GaAs technologies transformers may readily be achieved with total thicknesses of about 0.3 to 10 microns and with overall dimensions of about 50 to 500 microns. Generally speaking, capacitance between the layers is highly predictable and readily taken account of in circuit simulation using techniques well known in the art.

Two particular transformer designs having similar outer dimensions, one of them according to the prior art design of FIGS. 1A, 1B, 1C and the other according to the inventive design of FIGS. 2 and 3 were simulated and compared. In the inventive transformer, the self inductance of the primary winding was 5.96 nH (nanoHenries) and the self inductance of secondary winding was 5.93 nH, the coupling coefficient of two windings was approximately 0.9. In the prior art transformer, the self inductance of primary winding was 2.57 nH (nanoHenries) and the self inductance of secondary windings was 2.08 nH, the coupling coefficient of two windings was approximately 0.7. Thus, using the invention, a coupling coefficient of at least 0.8 is readily obtainable. The advantages in the inventive transformer of higher self inductance higher coupling coefficient and more equal self inductances are readily apparent.

The invention may be particularly valuable in GaAs integrated circuits since the provision of multiple metallization layers may be particularly expensive in GaAs integrated circuits, and in many cases the number of metallization layers available and is limited to just two.

Typical embodiments, fabricated in GaAs, may have spirals with diameters of about 50 to 300 microns. Also in GaAs embodiments, the two metallization layers together with an interposed insulating layer may have a total thickness or separation of about 0.3 to 10 microns. Using such techniques coupling coefficients of 0.9 or better are readily achieved.

Other topologies devices could also be used to construct embodiments of the invention using the appropriate fabrication arrangements. For example, the invention is not limited to square shape spirals but other shapes such as circles or polygons may be used in their place.

The embodiments described above are exemplary rather than limiting and the bounds of the invention should be determined from the claims. Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims. 

1. A transformer circuit consisting essentially of a first metallization layer; a second metallization layer; a plurality of conducting vias connecting the metallization layers and a plurality of further layers other than metallization layers wherein: a first winding is formed substantially consisting of a first conducting spiral formed in the first metallization layer, a first substantially radial conductor formed in the second metallization layer and a first via connecting the first conducting spiral to the first substantially radial conductor; and further wherein: a second winding is formed in proximity to the first winding, the second winding substantially consisting of a second substantially radial conductor, a gapped spiral formed in the second metallization layer, a plurality of short conductors in the first metallization layer, the short conductors bridging across the substantially radial conductors and a plurality of other vias connecting the gapped spiral to the short conductors.
 2. The circuit of claim 1 wherein: further layers comprises a GaAs substrate.
 3. The circuit of claim 2 wherein: the spirals have diameters of about 50 to 500 microns.
 4. The circuit of claim 2 wherein: the transformer circuit has a total thickness of about 0.3 to 10 microns.
 5. The circuit of claim 1 wherein: the first and second windings have a coupling factor higher than 0.8.
 6. The circuit of claim 1 wherein: the self inductances of the first and second windings are substantially equal.
 7. The circuit of claim 2 wherein: the first and second windings have a coupling factor higher than 0.8.
 8. The circuit of claim 2 wherein: the self inductances of the first and second windings are substantially equal. 